Past studies on the mechanisms of electron transfer by c-type cytochromes have yielded considerable information and have led us to formulate a number of interesting and potentially significant questions. The ultimate goal of this work remains to understand the mechanism of coupling biological electron transfer to energy conservation. However, to fully understand this process a number of related issues must be addressed which bear on both biological electron transfer and the general area of protein-protein interactions. Thus, an integral part of elucidating the mechanism of biological electron transfer focuses on the chemical and structural aspects of the interaction between two proteins which give these reactions their high specificity and rapid rates. The studies proposed here are designed to address both the question of the mechanisms of biological electron transport and the nature of protein-protein interactions. Specifically we propose to investigate the interactions of cytochrome c and cytochrome b5, two proteins for which considerable structural information is available. Further, we are proposing experiments designed to elucidate the surface structure of cytochromes in solution and to use this information in understanding protein-protein interactions. We anticipate that information derived from these studies will bear not only on the mechanism of biological electron transfer but will provide information useful in elucidating other phenomena such as interactions between antigens and antibodies, proteins and cell surface receptors and proteins and nucleic acids.